1. Field of the Invention
The invention relates to a circuit, referred to below as an input circuit, for detecting input signals, and further relates to an input circuit unit having at least one input circuit as well as to a method for operating the input circuit. As an input circuit that can be made redundant, the input circuit makes possible single operation or redundant operation, i.e., operation together with a further input circuit, and thus a redundant or non-redundant detection of the respective input signal.
2. Description of the Related Art
Input circuits and redundant input circuits are known per se. In conventional input circuits, the relevant input signal is detected via inputs permanently designed for a respective measurement range, to which only digital or analog input signals can be fed. To make analog inputs for current measurement redundant, such as 4 to 20 mA, a two-pole input circuit is used.
EP 2 461 221 A2 discloses a control device with a circuit for processing signals of different sensor types. Here, two sensor types which can be connected deliver either analog current signals or PWM signals. In order to provide a reference potential required for evaluating such signals, the circuit comprises a pull-up resistor and a pull-down resistor. Both can be activated by switches which can be actuated by a microcontroller. The measurement current flowing into the circuit is guided via a low-pass filter to two input channels of the microcontroller simultaneously, and the microcontroller evaluates the signals on that input channel which belongs to the previously activated resistor.
US 2012/0280327 A1 and U.S. Pat. No. 6,147,520 disclose conventional circuits in which a single resistor can be bridged by means of a switch connected in parallel in order to adjust a respective overall resistance.